Semiconductor image sensing devices are finding widespread application with the increased use of digital still cameras and digital video cameras. Charge coupled device (CCD) technology has hitherto dominated the market for such imagers. The present CCD imagers have a number of good features including high image quality and low noise characteristics; however, there are also several limitations which are inherent to CCD's. The specialized process utilized to produce CCD imagers is not well suited to forming driving and processing circuitry used in conjunction with the CCD image sensing array. Thus, it is necessary to implement those functions on secondary integrated circuit chips, which increases the complexity and cost of the CCD imaging system. The sensitive clocking requirements of CCD's adds further complexity and results in relatively high power consumption for CCD imaging systems.
An alternative to CCD imaging which is gaining popularity involves forming image sensors using complementary metal-oxide silicon (CMOS) processing. CMOS image sensors are advantageous in that the CMOS process allows for the inclusion of circuits for image processing and the like, allowing a high level of product integration to enable virtually all electronic camera functions to be integrated on a single chip. Image sensors manufactured in CMOS can also be made relatively inexpensively and facilitate significant power savings compared to CCD sensors. These are particularly important issues for portable consumer applications.
There have been some difficulties with CMOS image sensors relating to the image quality that can be obtained, and one source of the quality difficulties arises from the way in which pixel signal values are sampled in the imaging circuit. One form of CMOS image sensing involves pre-charging a circuit node with a reference voltage, exposing the node to image forming light to allow charge to leak from the node through light-induced leakage current for a predetermined period, and measuring the difference in voltage level at the node caused by the light exposure. The image sensing therefore requires sampling of the node voltage before and after exposure to light to determine the voltage level difference. Known sampling circuits to accomplish that end have some drawbacks arising from non-linearity in the sampling circuitry and limitations of the CMOS components which can result in, for example, fixed pattern noise (FPN).